The present invention relates to a phase shifter circuit, and, more particularly, to a phase shifter circuit used in a mixer or a modulator of radio communication apparatus, such as a cellular telephone.
FIG. 1 is a schematic circuit diagram of a conventional phase shifter circuit 10.
The phase shifter circuit 10 comprises a first differential amplifier 11 that receives an input signal (analog frequency signal) Sin and generates first and second phase shift signals S1, S2 having a phase difference of 180 degrees (e.g. 0° and 180°) with each other and a second differential amplifier 12 that receives an input signal Sin and generates third and fourth phase shift signals S3, S4 having the phase difference of 180 degrees (e.g. 90° and 270°) with each other. FIG. 3 is a graph showing the relationship between frequency and phase in each of the phase shift signals. As shown in FIG. 3, each of the phase differences between the phase shift signals S1, S3, between the phase shift signals S2, S3, between the phase shift signals S2, S4 and between the phase shift signals S4, S1 is 90 degrees. The graph shows that the phase shifter circuit 10 holds the phase differences of the respective phase shift signals S1 to S4 at 90 degrees at any frequency.
FIG. 2 is a graph showing the relationship between frequency and amplitude in each of the phase shift signals. As shown in this graph, however, the conventional phase shifter circuit 10 matches the amplitude of the first and second phase shift signals S1, S2 and the amplitude of the third and fourth phase shift signals S3, S4 only at a certain frequency (f0).
The phase shifter circuit 10, for example, as shown in FIG. 4, is applied to a mixer circuit 20 for a radio communication apparatus which switches a plurality of IF frequencies. A phase shifter circuit 10a receives an intermediate frequency signal IFin as the input signal Sin and generates first to fourth intermediate frequency signals. A limit amplifier 21a for matching the amplitude of the first to fourth intermediate frequency signals at a plurality of frequencies is connected to the phase shifter circuit 10a. A phase shifter circuit 10b receives a local oscillation signal LOin as the input signal Sin and generates first to fourth local oscillation signals. A limit amplifier 21b for matching the amplitude of the first to fourth local oscillation signals at a plurality of frequencies is connected to the phase shifter circuit 10b. Accordingly, the first to fourth intermediate frequency signals and first to fourth local oscillation signals whose amplitudes are balanced are supplied to modulation mixers 22a, 22b. However, use of the limit amplifiers 21a, 21b increases the circuit area and power consumption of the mixer circuit 20.